1. Field of the invention
The resent invention relates to a hydraulic control system for an automatic transmission.
2. Description of Related Art
Typically, automatic transmissions for automobiles comprise mechanical transmission devices or mechanisms and torque convertors. Available engine torque from a crankshaft is multiplied and transferred first by a torque convertor and subsequently by a mechanical transmission. Such a mechanical transmission includes a planetary gearset consisting of a sun gear, a ring gear and pinions which changes the path of power flow for gear shift. Specifically, in order for mechanical transmission to change gears or the path of power flow, the automatic transmission is provided with friction coupling elements, such as clutch mechanism for connecting and disconnecting the transmission of power to specific gears of the mechanical transmission and braking elements for braking specific gears of the mechanical transmission. These friction coupling elements and braking elements are selectively actuated by hydraulic control so as to perform gear shifts.
In the automatic transmission, if a shifting time which refers to a time necessary to complete a gear shift is too short, in other words, if the friction coupling element is abruptly actuated by high hydraulic pressure, the automatic transmission causes what is called "shift shock." Contrary, if the shifting time is too long, i.e. if the friction coupling element is slowly actuated by a high hydraulic pressure, there occurs a deterioration in the quality of driving. For that reason, in the hydraulic control of automatic transmission, the pressure of oil is typically controlled so as to complete gear shifts over specified shifting times according to driving conditions.
The shifting time is governed by torque transferred through a friction coupling element by which a specific gear shift is made and a coupling force of the friction coupling element, i.e. an oil pressure applied to the friction coupling element. Specifically, the greater the torque transferred through the friction coupling element, the longer the shifting time that a gear shift effected by the friction coupling element takes. On the other hand, with an increase in oil pressure, the shifting time becomes shorter. Consequently, controlling the oil pressure suitably in conformity with the torque transferred through the friction coupling element enables the automatic transmission to cause a specific gear shift achieved by the friction coupling element over a target or intended shifting time.
As it has been proved that a torque transferred through a friction coupling element related to a gear shift during shifting is a resultant force from a torque transferred to a transmission gear mechanism and an inertia caused by a change in rotational speed of a power transfer line to the transmission gear mechanism. That is, since a drop in rotational speed occurs to the turbine shaft of the torque convertor which is on the input side of the transmission gear mechanism, the power transfer line imparts to the friction coupling element the force of inertia in the same direction as torque of the turbine shaft during a shift-up and, on the other hand, in the direction opposite to torque of the turbine shaft during a shift-down. On that account, the hydraulic control system is adapted to control an oil pressure according to a torque transferred to the transmission gear mechanism and the force of inertia of the power transfer line to the transmission gear mechanism so as to enable the automatic transmission to cause a gear shift over a target shifting time. Hydraulic control systems of this kind are known from, for instance, Japanese Unexamined Patent Publications Nos. 3-249468 and 4-72099.
Experiments conducted by the inventors of this application led to the conclusion that hydraulic control systems of this kind are hard to cause the automatic transmission to achieve gear shifts over a target shifting time.
As shown in FIG. 11, the experimental results prove a comparatively adequate correlation which has a friction coefficient of correlation of 0.96011 between the inertia component pressure and angular deceleration .omega.'. On the other hand, as shown in FIG. 11, the correlation between the torque component pressure and the input torque Tt has a coefficient of correlation of 0.788101, alluding that the torque component pressure is less correlated with the input torque Tt. According to the experimental results, there is a linear relation between the inertia component pressure Tci and angular deceleration .omega.'. However, it is hard to say there is a linear relation between the torque component pressure and the input torque Tt. In FIG. 12, the measured values seem to be on a curve. If the equation (1) represents properly the correlations of line pressure with angular acceleration and a torque, they must have a coefficient of correlation of 1 (one) and all of the measured values must be exactly on straight lines in FIGS. 11 and 12.
It is apparent from the above discussion regarding the estimation of a target pressure P that the equation (I) is not always precise in order to estimate the target pressure P which is sufficiently accurate for achievement of a gear shift over an intended shifting time.
According to a conclusion derived by the considerations by inventors of this application, while the kinetic friction coefficient of a friction coupling element is considered to change according to an interfacial pressure or a relative speed between drive and driven members of the friction coupling elements, the prior art hydraulic control system is designed and configured on condition that the kinetic friction coefficient of a friction coupling element is constant and, consequently, produces changes in the target shifting time due to changes in the kinetic friction coefficient. In the light of the above considerations, it concluded that controlling the line pressure according to changes in the kinetic friction coefficients of friction coupling elements cause gear shifts achieved exactly over intended shifting times, respectively. Specifically describing, in the prior art hydraulic control system for an automatic transmission, a line pressure P, which in turn refers to a target shifting pressure, is regarded to be given by the following equation (I): EQU P=A.multidot..omega.'+B.multidot.Tt+C (I)
where .omega.' the angular acceleration, i.e. the force of inertia;
Tt is the torque transferred to the transmission gear mechanism; and PA1 A, B and C are constants, respectively.
If the constants A, B and C can be known based on a plurality of measurements of these angular acceleration .omega.' and torque Tt and a line pressure at which the automatic transmission actually achieves a gear shift over an intended shifting time, a target pressure P is obtained for various angular acceleration .omega.' and torque Tt. Keeping the line pressure at the target pressure P forces the automatic transmission to achieve the gear shift over the intended shifting time.
The inventors made a survey of these factors for gear shifts achieved over the intended shifting time and a multiple regression analysis of 64 sets of angular acceleration .omega.', input torque Tt and line pressure P was done to determine the constants A, B and C which leads to minimum errors. According to the equation (I), a part of the whole hydraulic pressure corresponding to angular deceleration .omega.', which is referred to as an inertia component pressure, is proportional to angular deceleration .omega.'. Further, a part of the whole hydraulic pressure corresponding to an input torque Tt, which is referred to as a torque component pressure, is proportional to an input torque Tt. In other words, there must be a linear relation both between the inertia component pressure and angular deceleration .omega.' and between the torque component pressure and the input torque Tt. The experimental results which were obtained in the way described above are shown in FIGS. 11 and 12.